The invention relates to a method for controlling the temperature of print bearing surfaces during printing in rotary printing machines.
During printing in a multicolor offset printing machine, in particular, a sheet-fed offset printing machine, a plurality of phenomena are known that affect the position or the geometry of the printed images of the individual color separations, i.e., the color separations for black, magenta, cyan, and yellow, from which the printed image is composed. As a result of these phenomena, the register of the individual color separations, from whose superposed printing, the printed image arises, can no longer be set correctly on the overall sheet.
Round printing manifests itself for example as a deviation of shape in the printed images of different separations, resulting, in particular, as curvatures of lines running transverse to the direction of printing.
In addition, narrower/broader printing describes the differences in the print widths, seen over the sheet length. Given an increase of this effect in the print direction, the deviation of the print widths is generally greatest at the rear edge of the sheet. In the rear printing units, in relation to the direction of sheet travel, of a series printing machine, there is a tendency for printing to be narrower in comparison with the color separation of the first printing unit.
The phenomenon of shorter/longer printing designates the differences in the print length. Because the front edge of the sheet is adjusted so as to fit precisely, this effect is likewise seen in registration differences with an increasing tendency in the direction towards the rear edge of the sheet. In comparison with the color print from the first printing unit, in general, printing is shorter in the rear printing units as seen in the direction of sheet travel.
Given printed material that is approximately 135 g/m2, and that passes through four successive printing units, the deviation in registration caused by the three stated phenomena lies in the range of up to half a screen frequency, and causes color displacements in multicolored screen surfaces that are clearly perceptible in the print. In addition, losses of print quality with respect to depth of detail and delineation are known, or are to be feared. With the increasing use of CtP (computer-to-plate) apparatuses, a worsening of the problem can be expected. Given precisely registered printing forms, the time that is saved with CtP will be more than offset by the time required for manual corrections of each color separation.
The causes of the positional deviations or of the geometric deviations that occur are of very widely varying types. The absorption of dampening solution by the sheet during its passage through the respective print gap or nip contributes decisively to the deformation of the sheet during the passage through the individual print units. In addition, the print pressure set in the print gap plays a further significant role. The tack of the ink transferred onto the surface of the blanket can likewise result in a significant deformation of the sheet during its passage through the print units.
Grippers that are arranged centrically over the width of the printing unit, and that guide the sheet at the circumferential surface of cylinders or drums, can locally loose hold of the sheet. This results in a non-flat seating of the sheet on the surface of the cylinder that guides the sheet, for example the counter-pressure or impression cylinder of a printing unit. Under the pressure prevailing in the print gap, a rolling out of this deformation that arises takes place in the respective sheets to be printed. Mechanical deformations, such as for example, cylinder deflection of the cylinder that guides the paper between the side walls of the printing units, as well as winding differences in the individual printing unit cylinders, can also be significant.
Further influencing factors can include: the format to be printed, the rigidity of the paper-guiding cylinder, the number of successive printing units, as well as the position of the drive. Whether the rotary printing machine uses a perfecter is also an influencing factor. Finally, the printing speed is a factor that influences the sheet: deformation.
Regarding the printing material, significant factors include the density of the printing material, its porosity, the direction of travel, and whether the material is long grain or short grain. Furthermore, the water absorption characteristic, which has a striking influence on the deformation, plays a considerable role. The subject to be printed, the surface covering, and the respective level of coloration should also be mentioned. Besides the tack of the ink, other important factors include the separation behavior of the printed material from the respective ink-bearing surface of the blanket cylinder, as well as the setting of the pressure and of the dampening, of the respective printing unit.
Up to now, it has been attempted to counteract these phenomena by reinforcing the cylinders in the printing units of the sheet-processing printing machine against deflection. In addition, the plate and impression cylinders were previously classified into various diameter tolerances, and were constructed so that the diameter of the impression cylinders in the rear printing units had a tendency to increase, while the diameter of the respective plate cylinders decreased in the direction towards the rear printing units.
In straight-printing and perfecting-printing machines, cylinder jackets in corresponding gradations have also been used. In case of problems during the printing job, the jackets are replaced, or are simply exchanged at the respective cylinders. An attempt has been made to compensate for a shorter printing by using calibrated underlay sheets under the printing plates. In addition, it has been attempted, at considerable expense, to deform the print plates at the rear edge in the circumferential and lateral direction on the respective print form cylinder, through their mounting.
It is accordingly an object of the invention to provide a method for influencing the geometry of a print bearer that is held on a print form cylinder of a multicolor rotary printing machine which overcomes the above-mentioned disadvantageous of the prior art apparatus and methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for controlling the temperature of print bearing surfaces during printing in a rotary printing machine, that includes steps of: providing a plurality of printing units having printing form cylinders with circumferential surfaces; in each one of the plurality of the printing units, exchangeably fastening a print bearer on a respective one of the circumferential surfaces of the print form cylinders; in each one of the plurality of the printing units, using at least one inking form roller to ink the respective print bearer; and in each one of the plurality of the printing units, during printing, controlling a temperature of a color separation-guiding surface of the respective print bearer to compensate for a printing effect selected from the group consisting of narrower/broader printing and shorter/longer printing that respectively arises during passage of printed material in the one of the plurality of the printing units.
In other words, the temperature of the print bearer surfaces are controlled during printing in rotary printing machines in which the print bearers are fastened in an exchangeable fashion on the circumferential surfaces of print form cylinders, and the print bearers are inked via inking form rollers. The temperature of the surfaces of the print bearers guiding the color separations can be controlled in each individual printing unit, for compensating for narrower/broader printing and/or for shorter/longer printing that arises respectively during the passage of the printed material in the individual printing unit.
The advantages resulting from the solution enables, in real time during the printing process, the print length and print width to be individually influenced per individual printing unit of a rotary printing machine. Manual adjustments that were previously carried out according to the skill and professional experience of the printer at the sheet compensators for the correction of round and narrow printing can now be entirely omitted by controlling the temperature of the print bearer surfaces. Controlling the temperature, individually per printing unit, of the color separations located on the surfaces of the print bearers, can on the one hand take place on the upper side thereof, via a corresponding controlling of the temperature of the inking form rollers and of the dampening form roller, or of the inking unit, as well as from the underside of the printing forms via a corresponding controlling of the temperature of the cylinder jacket surface. With this procedure, a temperature profile can be produced at the printing form surface, which because the expansion characteristic of the printing form surface is independent of direction, permits both an elongation of the printing form, and seen in the direction of print rolling, a broadening of the printing form.
In accordance with an added feature of the invention, a uniform temperature or a printing system temperature can be set economically by controlling the temperature of the ink-bearing inking form rollers.
In addition to controlling the temperature of the print form surface that guides the color separations, the temperature of the print form cylinders that respectively receive the print bearers on their circumferential surfaces can be controlled, for example, via a an external heat supply.
Controlling the temperature of the print form surface via a temperature profile that can be predetermined at the jacket surface of the print form cylinder at the underside of the printing form allows the predetermination of the temperature level, and also the distribution of temperature, in the circumferential direction and/or lateral direction of the printing form, individually per printing unit.
As a rule, print bearers or printing forms made of aluminum have a coefficient of thermal expansion of approximately 24 xcexcm/mK, and therefore react fairly sensitively to changes in temperature, which can be exploited in order to produce direction-independent changes in elongation at the print bearer. The temperature in the transition region at the surface of the color separation of the print bearer can be set in the range between 10xc2x0 C. and 60xc2x0 C., in which range it is possible to achieve particularly advantageous pressure and ink transfer characteristics. If the method is used in rotary printing machines in which a plurality of individual printing units are arranged one after the other in series, then, given a constant temperature of the printing form per individual printing unit and the concomitant enlargement of the print form surface, with exploitation of the tendency towards narrow printing in the rear printing units of a multicolor rotary machine, the effects of the shorter/longer printing and of the narrower/broader printing can be simultaneously compensated.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for controlling the temperature of printing form surfaces during printing, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.